The increasing use of Doppler-based radar systems to provide information used in the control of critical industrial equipment places greater emphasis on the reliability, and necessarily, the continuous operability of such systems.
One particular application involving the use of radar systems in the control of critical equipment is in railroad classification yards. More specifically, radar systems are presently being utilized in hump yards where individual and groups of rail cars travel through a series of switches and retarder sections in order to redistribute themselves to their designated trains. The combination of computer controlled switching and retarding operations provides control of both the direction and the speed of the rail cars.
As the cars approach the individual retarder sections, magnetic wheel sensors operate to determine their presence. Upon receiving an appropriate indicator signal from the wheel sensor, the dedicated radar system operates to monitor the speed of the rail car and to continuously relay this information to the hump yard control system computer. In response, the control system computer continuously keeps track of the position and speed of the car. In turn, it controls the switches to send the car to the proper spur, or parallel rail make-up line, where that particular train is being assembled, and it adjusts the retarding force applied to the wheels of the car while it travels through each retarding section. These continuous adjustments to the retarding force are designed to slow the cars from the standard speed established by gravity over the hump to an optimal exit speed.
Upon exiting the primary retarder section, the cars travel through a secondary retarder section and/or then through the series of switches which direct the cars to the proper parallel rail make-up line where they engage and couple with the next-in-line car in the designated train. Ideally, the desired speed at which proper coupling occurs without damaging the rail car's coupling mechanisms or internal cargo is about four miles per hour.
A failure in any portion of the hump yard control system, and particularly in the individual radar systems, can of course result in a collision, and even rail car derailment. The expense of repair and the system downtime due to such an accident is particularly costly. Thus, there is a distinct need identified to minimize, or preferably eliminate altogether, the potential occurrence of such radar system failures.
In view of this need, various methods and apparatus for testing the operability of Doppler-based radar systems have evolved. One particular method presently utilized to test hump yard radar systems, as well as other radar systems, involves interposing a test signal directly into the radar system's amplifiers and/or counting circuitry, thus bypassing the antenna, receiving diode and other of the critical microwave components. In this method, the output of the radar system under test is intermittently compared to the known input signal by manual observation. This method of testing radar systems is thus adequate to only test the system's amplifiers and/or internal counting circuitry. However, the method falls short of determining the overall operability of the entire system; most importantly, with respect to the microwave components, as mentioned above. Surveys have found that typically it is these microwave components that incur the greatest number of critical failures. The fact that these components are not subjected to a test for operability by this prior system is a serious shortcoming.
Indeed, a failure in any of the microwave components of a radar system can result in a false signal reading of 2-3 miles per hour. This false signal reading in one common occurrence is the result of signal noise from the surrounding radar systems and other sources throughout the hump yard. Unfortunately, these false signal readings not only prevent operation at the optimal exit speed for the rail cars, but the magnitude of the error compared to the optimal speed is enough to not only risk substantial damage to the couplers and undercarriage of the car, but cause the derailment, as mentioned above. In other words, with a microwave component failure, the hump yard control system computer incorrectly opens the retarders allowing the cars to proceed uncontrolled through the section, and thus maintain the excessive speed that causes the problem.
In addition to the self test method described above, additional methods and apparatus are known which are capable of testing the operability of the entire radar system. One such method uses the moving tines of a tuning fork to simulate a moving target. In this manner, the entire radar system can be evaluated by monitoring the resulting output of the system. An output reading corresponding to the frequency of the tuning fork indicates the proper operation of the overall radar system, including its microwave components.
Although this particular method is thus capable of testing the entire radar system, its use in hump yards, as well as in numerous other systems utilizing Doppler radar systems in the control of critical industrial equipment, is of limited utility. Specifically, the use of tuning forks requires the relatively frequent presence of personnel in the hump yard. As will be recognized, this makes the test operation labor intensive and increases the operation cost significantly. In addition to increasing the operating costs of the hump yard, it introduces the factor of human error, which can lead to the same deleterious results that are being attempted to be overcome.
An apparatus described in U.S. Pat. No. 4,656,481 to Mawhinney utilizes a separate, outside generated test signal for checking and calibrating a radar system. This instrument utilizes a modulation device to generate an acoustic signal of known frequency. The output of the radar unit under test is manually monitored for the proper visual and acoustic outputs. When used as a medical device, the '481 patent discloses the use of the piezoelectric sounder for producing pulses indicative in amplitude and duration of a heartbeat. These pulses create the acoustic signal which is manually monitored by the technician performing the calibration procedure. Thus, while the '481 patent discloses the use of a modulated test signal, there is no method of self-generating the test signal with the radar system under test, modulating the frequency of the test signal, or of monitoring the output of the system remotely to determine its overall operability.
Therefore, each of these prior art methods and apparatus are of limited utility in testing radar systems, and particularly the operability of the system components, that are used to remotely control critical industrial equipment. For example, these prior art methods may be appropriate to test traffic speed measurement radar systems, but not appropriate to be used as a highly reliable and stand alone system for monitoring and controlling industrial equipment. Indeed, it is not a problem for a police officer to periodically ring a tuning fork and visually observe and compare the resulting speed readout to the standard of the tuning fork. On the other hand, in cases where radar systems control critical industrial equipment, and the tests must be performed by remote control, preferably each time the system operates and without introducing the factor of human error, the prior art methods fall short. At the same time, holding down the operational costs, is an important consideration where the prior art systems are also sorely deficient.
Thus, there is a need identified for a highly efficient and economical Doppler-based radar system having a self test circuit for use in remotely determining the overall operability of the entire radar system, including the microwave components, as well as the over-all industrial control application.